Method and apparatus for thermally processing polyester pellets and a corresponding pellet preparation

ABSTRACT

Method and apparatus for thermally processing polyester pellets, e.g., polyethylene terephthalate pellets, in order to achieve a partial crystallization, whereby the polyester melt is fed to an underwater pelletizer and pelletized, the pellets obtained are fed to a water/solids separating device and the dried pellets are fed at a pellet temperature of greater than 100° C. to an agitation device that the pellets leave at a pellet temperature of over 100° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.10/962,614, filed Oct. 13, 2004, which claims priority under 35 U.S.C.§119 of German Patent Application No. 103 49 016.7-43, filed Oct. 17,2003, and German Patent Application No. 10 2004 021 595.2, filed May 3,2004, the disclosures of which are expressly incorporated by referenceherein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for thermally processing polyesterpellets in order to achieve crystallization and to the polyester pelletpreparation produced thereby.

2. Discussion of Background Information

Polyethylene terephthalate, hereinafter also referred to herein as PETfor short, is a polyester with repeating ester groups.

PET can be present in different structures, namely in amorphous or incrystalline or partially crystalline form. Amorphous PET is mostlytransparent, and crystalline PET is opaque or white. As with allthermoplastics that can be present in amorphous or crystalline form, a100% degree of crystallization is also not possible with PET. Only aportion of the structure of the PET is able to orient itself, i.e., tocrystallize, and PET includes crystalline and amorphous regions whichalternate. Therefore, PET is always referred to with respect to partialcrystallinity. An approximate 50% degree of crystallinity can beachieved with PET in order to prevent the pellets or granules fromsticking to one another. This means that in this state about half of themolecule chains have oriented themselves to one another, and thus havelaid themselves parallel next to one another or have wound themselves ina circular manner. The interactions (van der Waals forces) between themolecular chains therefore inevitably become greater in the partiallycrystalline regions. The chains thus mutually attract one another andthus gaps between the molecules become smaller.

As a thermoplastic, PET can be molded at temperatures of 250 degree. C.The molecular chains then become so mobile that the plastic melts and aviscous mass results that can be made into virtually any desired shape.When it cools, the molecular chains refreeze, and the plastic solidifiesin the desired shape—a simple principle that can be repeated multipletimes. This method is also used, e.g., in the production of PET bottles.So-called preforms are produced in a first step. As a precursor of thePET bottles, these preforms already have a finished screw thread. Inorder to obtain proper bottles, they are again softened at 100° C.,stretched with compressed air and blown to produce a bottle (stretchblow process).

The production of crystallized PET in pellet form hitherto comprisedextensive and complicated fluidized bed methods that required largeinvestments and high operating costs, such as DE 198 48 245 A and itsfamily member WO 00/23497, which are incorporated by reference herein intheir entireties.

PET pellets have to be crystallized at a temperature below that at whichthe material becomes sticky in order to prevent the pellets coalescinginto a solid mass that can barely be processed. Although the meltingtemperature of the crystallized polyester is not reached until 240 to250° C., it can already become sticky before crystallization attemperatures above approx. 70° C.

In so far as continuous methods for producing dry PET pellets are known,in general they require very large installations, since longcrystallization times are necessary.

Thus, for example, U.S. Pat. No. 5,532,335, which is incorporated byreference herein in its entirety, is directed to a method for thermallyprocessing polyester pellets in which the pellets are introduced into aprocessing vessel and a liquid medium is also introduced into thisprocessing vessel, whereby the pellets and the liquid medium are mixedtogether. Pressurized water or so-called superheated water is usedhereby as a liquid medium in the proposed process. The boilingtemperature can easily be controlled by changing the pressure in thereactor vessel. In an exemplary embodiment polyester pellets areprocessed at 120°-182° C. The water is introduced at 160° C., kept inthe liquid state and the pellets are added as long as the pressure inthe reactor unit is kept at 7 kg/cm² or higher. It is evident that sucha method is extraordinarily expensive and therefore can barely beconducted economically.

Known methods that work with an aerodynamic processing also have theserious disadvantage that they use a large amount of inert gases. Theenergy and processing costs are also too high here for a practicallarge-scale application.

In order to sufficiently crystallize the material in the prior art itwas therefore always necessary to add sufficient external energy or heatto the crystallization process. These cited problems have hithertohampered PET recycling.

SUMMARY OF THE INVENTION

The present invention relates to a method for crystallizing PET pelletsthat is possible without the addition of external energy or heat andthat does not require long dwell times. The present invention furtherrelates to the crystallized PET pellet preparation and to thatpreparation produced according to the method of the invention.

The present invention relates to a method for thermally processingpolyester pellets in order to achieve partial crystallization,comprising feeding polyester melt to an underwater pelletizer andpelletizing the polyester melt in the underwater pelletizer to obtainpellets, feeding the pellets to a water/solids separating device to drythe pellets, feeding the dried pellets at a pellet temperature ofgreater than 100° C. to an agitation device, and removing pellets fromthe agitation device at a pellet temperature of over 80° C.

The present invention also relates to an apparatus for carrying out amethod for the thermal processing of polyester pellets in order toachieve a partial crystallization of the pellets, the apparatuscomprising a melt pump; a screen changer; an underwater pelletizer; awater/solids separating device; and a conveyor device for transportingpellets, the conveyor device being arranged downstream of saidunderwater pelletizer and said water/solids separating device, saidconveyor device being constructed and arranged to agitate pellets andcrystallize pellets during transport through specific heat of thepellets.

The present invention also relates to polyester pellets, preferably PETpellets that are crystallized at least to 40% by the specific heatpresent in the pellets. That specific heat results from the pelletformation. The pellets have an outermost layer spheroylitic structurewhich is equal or smaller than that at the center. This characteristiccan be distinguished by polarization-contrast optical microscopy and bythe change in visible pellet character from translucency to opacity. Inparticular, the polyester pellets have a degree of crystallization attheir centers which is at least as great as the degree ofcrystallization of their outermost layers. The pellets also have anacetaldehyde content between 0.5 and 100 ppm.

The method can further include flowing a fluid around the pellets duringagitation of the pellets in the agitation device.

The pellets can be in a form of a pellet layer, and further comprisingflowing a fluid around the pellet layer during agitation of the pelletsin the agitation device.

The pellets can be fed to the agitation device at a pellet surfacetemperature of over 110° C.

The pellets can be conveyed from the underwater pelletizer to thewater/solids separating device with hot process water.

The process water can have a temperature of 98° C.

The thermal processing leading to partial crystallization can utilizespecific heat present in the pellets.

The polyester pellets can comprise polyethylene terephthalate pellets.

The conveyor device can comprise a conveyor channel.

The conveyor device can comprise a vibrating conveyor device.

The vibrating conveyor device can comprise a conveyor channel.

The conveyor device can a plurality of spaced apart dams distributedover the length of the conveyor device or conveyor channel, each of saidplurality of spaced apart dams causing a damming up of material.

The conveyor device can be surrounded at least in part by a housing.

The water/solids separating device can comprise a centrifuge.

In other words, it is proposed that the PET starting material isextruded in an extruder at a suitable temperature. Subsequentlyimpurities are filtered out using, e.g., screen changer technology. Thepolymer melt is fed to an “underwater hot strike off pelletizingsystem,” referred to below as “underwater pelletization,” and processedinto pellets that due to the underwater pelletization have a ball shapeor a lenticular shape and have a high core temperature.

These PET pellets are conveyed via a conveyor line at high speed to awater/solids separation device, whereby hot water, preferably up to 98°C., is used as a flow medium. One important aspect for the effectivenessof the method according to the invention is relatively short conveyorpaths between the pelletization chamber and the water/solids separatingdevice. The PET pellets leave the water/solids separating device at acore temperature of 130-180° C., since it is ensured that the extrusiontemperature of the PET is maintained for as long as possible.

The pellets having this temperature are then subjected to an agitationwhereby the crystallization begins. This crystallization according tothe method according to the invention is determined by the specific heatand it is thus achieved that the product, i.e., the pellets, do notagglomerate and no longer stick to one another. This effect is alsoincreased in that the product to be crystallized has a ball shape or alenticular shape, and thus manages with the smallest possible contactsurfaces to one another.

The dwell time of the ball-shaped pellets in the agitation phase as thepellets pass through an agitation apparatus, such as a conveying device,is, e.g., 3 to 8 minutes and after this phase has been completed, up to40% and more of the PET pellets are crystallized and have a temperatureof greater than 100° C. The transport of the hot PET pellets into astorage silo or subsequent processing station is possible, since thepellets no longer stick together.

Another object of the invention is to propose a device with which theeffective agitation of the pellets is possible.

Preferably, a so-called crystallization channel is provided as theagitation device for the pellets. This crystallization channel isconstructed in a similar manner to a pellet conveyor channel, but, seenin the conveyor direction, is divided into successive chambers that areseparated from one another by dams. The crystallization channel hasvibration motors so that the pellets located therein are permanentlyagitated and thus can give off their intrinsic energy to other pellets.A rotation of the PET pellets takes place in the individual chambers anda sticking of the pellets is no longer possible.

With the method according to the invention and the apparatus accordingto the invention a gentle, economic and rapid crystallization of PETpellets is achieved.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure and the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the sole FIGURE of drawings by way ofnon-limiting example of exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the several forms of thepresent invention may be embodied in practice.

The invention provides a sufficient heat energy to the pellets withtheir formation so that the pellets have a sufficient heat capacity toallow post-formation “after-curing.” The “after-curing” causes the abovedescribed partial crystallization of the pellets. This crystallizationis made possible by the intrinsic heat energy present in the pelletcores so that this procedure can be called “direct crystallization.” Forexample, following extrusion of the polyester melt and pellet formationfrom the melt, the pellet surfaces can be cooled down in order toproduce dimensional pellet stability. However, if the pellets are notcooled down intensively, then enough heat energy exists in the pelletcores is sufficient to cause the surface temperature of the pellets toslowly rise and accomplish the desired thermal treatment causingcrystallization.

Related to the examples described, a hot plastic with a temperature offrom, for example 130 to 180° C. can be molded into pellets, e.g., PETplastic, by means of the above described underwater granulation. Whilethe pellets spend a dwell time in the water, their surface temperaturecools down to a value of approximately 110° C. The core temperature ofthese pellets, however, remains significantly higher. If the pellets arenow separated from the water, such as by a centrifugal dryer, thefurther comparatively intensive cooling of the pellets is interruptedbecause the pellets release their heat energy to surrounding air slowerthan they would to surrounding water.

After the dryer, the pellets are agitated for a period of time asdiscussed above. During an agitation dwell time of several seconds,preferably even several minutes, a slow heating up of the pelletsurfaces takes place through the distribution of the higher pellet coreheat energy to the pellet surfaces. The surfaces reach, for example,140°-150° C. whereby the desired effect of direct crystallizationoccurs. This crystallization is indicated visually by the color changeof the pellets, which change from a first glassy or translucentcondition into an opaque white coloring.

In the drawing, 1 is used to label a melt pump 14 and a screen changer12 to which a polyester is fed according to the arrow F.sub.1. At theoutlet of the screen changer 12, an underwater pelletizer 2 is providedthrough which pellets with a ball shape or a lenticular shape areproduced. These pellets are guided through a conveyor device to awater/solids separating device 3, e.g., a centrifuge, whereby theconveying is carried out by process water that preferably has atemperature of over 80° C. The pellets leave the water/solids separatingdevice 3 at a temperature of over 110° C. and are fed to a conveyorchannel 4 to which supply air can be fed at 5 that leaves the conveyordevice 4 at 6 and ensures a removal of the moisture. The conveyor device4 is embodied as a conveyor channel with dams 7 aligned crosswise withrespect to the conveyor direction and vibration motor 11, and thepellets leave the conveyor device 4 at a pellet temperature of over 100°C. and can be fed via a so-called pellet diverter valve 8 to an aftertreatment device 9 or a silo 10.

These pellets are crystallized at least to 40%, and can be handled.

Surface temperature of pellets according to the present invention can bemeasured by contactless infrared devices, such as those that are readilyavailable in the market, e.g., model “Raynger MX” of the brand “RAYTEC”,which model is pistol-shaped and with which a surface temperature can bemeasured over a distance of several feet.

Moreover, the surface temperature of the pellets can be estimated asbeing around 20° C. to 30° C. lower than the core temperature of thepellets, so that by measuring the surface temperature of the pellets, arough idea of the core temperature can be deducted. Accordingly,measurement of the surface temperature can provide an estimation of thecore temperature of the pellets by adding about 20° C. to 30° C. to themeasured surface temperature.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to an exemplary embodiment, it is understood that thewords which have been used herein are words of description andillustration, rather than words of limitation. Changes may be made,within the purview of the appended claims, as presently stated and asamended, without departing from the scope and spirit of the presentinvention in its aspects. Although the present invention has beendescribed herein with reference to particular means, materials andembodiments, the present invention is not intended to be limited to theparticulars disclosed herein; rather, the present invention extends toall functionally equivalent structures, methods and uses, such as arewithin the scope of the appended claims.

1. A method for thermally processing polyester pellets in order toachieve partial crystallization, comprising feeding polyester melt to anunderwater pelletizer followed by pelletizing the polyester melt in theunderwater pelletizer to obtain pellets having a ball shape orlenticular shape, feeding the pellets to a water/solids separatingdevice to dry the pellets so that the pellets have a pellet surfacetemperature of no less than 100° C. to no greater than 160° C., feedingthe dried pellets at a pellet temperature of greater than 100° C. to anagitation device, transporting and agitating the pellets on theagitation device to obtain partial crystallization utilizing specificheat present in the pellets, and removing pellets from the agitationdevice at a pellet temperature of over 80° C.
 2. The method according toclaim 1, further comprising flowing a fluid around the pellets duringagitation of the pellets in the agitation device.
 3. The methodaccording to claim 1, wherein the pellets are in a form of a pelletlayer, and further comprising flowing a fluid around the pellet layerduring agitation of the pellets in the agitation device.
 4. The methodaccording to claim 1, wherein the pellets are fed to the agitationdevice at a pellet surface temperature of over 110° C.
 5. The methodaccording to claim 4, further comprising flowing a fluid around thepellets during agitation of the pellets in the agitation device.
 6. Themethod according to claim 4, wherein the pellets are in a form of apellet layer, and further comprising flowing a fluid around the pelletlayer during agitation of the pellets in the agitation device.
 7. Themethod according to claim 1, wherein the pellets are conveyed from theunderwater pelletizer to the water/solids separating device with hotprocess water.
 8. The method according to claim 7, wherein the processwater has a temperature of 98° C.
 9. The method according to claim 1,wherein thermal processing leading to partial crystallization utilizesspecific heat present in the pellets.
 10. The method according to claim1, wherein the polyester pellets comprise polyethylene terephthalatepellets.
 11. An apparatus for carrying out a method for the thermalprocessing of polyester pellets in order to achieve a partialcrystallization of the pellets, said apparatus comprising: a melt pump;a screen changer; an underwater pelletizer; a water/solids separatingdevice; and a conveyor device for transporting pellets, said conveyordevice being arranged downstream of said underwater pelletizer and saidwater/solids separating device, said conveyor device being constructedand arranged to agitate pellets and crystallize pellets during transportthrough specific heat of the pellets.
 12. The apparatus according toclaim 11, wherein the conveyor device comprises a conveyor channel. 13.The apparatus according to claim 11, wherein the conveyor devicecomprises a vibrating conveyor device.
 14. The apparatus according toclaim 13, wherein the vibrating conveyor device comprises a conveyorchannel.
 15. The apparatus according to claim 11, wherein the conveyordevice comprises a plurality of spaced apart dams distributed over thelength of the conveyor device, each of said plurality of spaced apartdams causing a damning up of material.
 16. The apparatus according toclaim 12, wherein the conveyor device comprises a plurality of spacedapart dams distributed over the length of the conveyor channel, each ofsaid plurality of spaced apart dams causing a damming up of material.17. The apparatus according to claim 13, wherein the vibrating conveyordevice comprises a plurality of spaced apart dams distributed over thelength of the vibrating conveyor device, each of said plurality ofspaced apart dams causing a damming up of material.
 18. The apparatusaccording to claim 14, wherein the vibrating conveyor device comprises aplurality of spaced apart dams distributed over the length of thevibrating conveyor device, each of said plurality of spaced apart damscausing a damming up of material.
 19. The apparatus according to claim11, wherein the conveyor device is surrounded at least in part by ahousing.
 20. The apparatus according to claim 11, wherein thewater/solids separating device comprises a centrifuge.
 21. The apparatusaccording to claim 11, wherein the water/solids separating devicecomprises a centrifuge.
 22. A polyester pellet crystallized at least to40% by specific heat present in the pellet from its formation.
 23. A PETpellet crystallized by specific heat present in the pellet from itformation, wherein said pellet is obtained by a process comprising:forming hot PET into pellets with a core temperature of 130-180° C. byunderwater granulation; cooling the surface temperature of the pelletswith water to about 110° C., while the core temperature in the pelletsis higher; separating the pellets from the water; and allowing thesurface temperature of the pellets to rise to 140-150° C. to effectcrystallization of the pellets.
 24. A polyester pellet crystallized atleast to 40% by specific heat present in the pellets from in formation,which pellet has an outermost layer spheroylitic structure of a particlewhich is equal or smaller than at the center of the particle asdistinguishable by polarization-contrast optical microscopy.
 25. Thepolyester pellet according to claim 24, wherein the pellet has a degreeof crystallization at the center of a particle which is at least asgreat as the degree of crystallization of the outermost layer thereof.26. The polyester pellet according to claim 24, which has anacetaldehyde content between 0.5 and 100 ppm.
 27. The polyester pelletaccording to claim 26 which has an acetaldehyde content between 0.5 and70 ppm.
 28. The polyester pellet according to claim 27 which has anacetaldehyde content between 0.5 and 60 ppm.
 29. The polyester pelletaccording to claim 24 which has a heat of fusion of less than 50 kJ perkg.
 30. A method of producing a PET pellet, which comprises usingspecific heat present in the pellet from its formation to produce apellet with at least 40% crystallization and an acetaldehyde contentbetween 0.5 and 100 ppm.
 31. The polyester pellet according to claim 25,which has a heat of fusion greater than 50 kJ per kg.
 32. A PETpolyester pellet produced by direct crystallization and having an opaquewhite coloring.
 33. A PET polyester pellet according to claim 32 whereinthe crystallization is at least 40%.
 34. A PET polyester pelletaccording to claim 32 wherein during direct crystallization, the pelletsurface has been cooled from a maximum temperature of 160° C. toapproximately 110° C. and then allowed to heat to a surface temperatureof 140 to 150° C. by transfer of heat energy from the pellet core.
 35. APET polyester pellet according to claim 32 that has undergone a changein color from a first translucent condition to opaque, white coloringduring its direct crystallization.
 36. The method according to claim 1wherein the pellet surface temperature rises to 140° to 150° C. duringthe partial crystallization step.